1. Field of the Invention
The present invention relates to a switching power supply apparatus, such as a resonance-type power supply apparatus or a half-bridge-type power supply apparatus, for example, which outputs a predetermined voltage signal by causing a plurality of switching elements to be alternately turned on and off.
2. Description of the Related Art
In the past, various kinds of switching power supply apparatuses have been developed, each of which outputs a predetermined voltage signal by causing a plurality of switching elements to be alternately turned on and off. For example, in a switching power supply apparatus that utilizes a PWM method of a half-bridge-type converter, a time ratio is adjusted at a constant switching frequency, thereby obtaining a desired output voltage signal. However, in such a switching power supply apparatus in which a plurality of switching elements are alternately turned on and off, if there is a time period when a plurality of switching elements are simultaneously turned on, a large short-circuit current flows, and there is a possibility that the power supply apparatus will be destroyed. Therefore, a dead time is provided in which all of the plurality of switching elements are turned off.
Since such a dead time is provided, a switching power supply apparatus disclosed in WO2005-076447 includes a first switching element and a second switching element that are alternately turned on and off, and the change of magnetic flux of a transformer due to the turning off of the first switching element is used as a trigger to turn on the second switching element. In addition, the change of magnetic flux of a transformer due to the turning off of the second switching element is used as a trigger to turn on the first switching element. By performing such switching control as described above, the first switching element and the second switching element are prevented from being simultaneously in an on state.
However, in the above-described switching power supply apparatus disclosed in WO2005-076447, since the on-time of the second switching element is determined by a time constant circuit including a resistor and a capacitor, a switching frequency varies and a switching noise occurs in a wide range in response to the variation of the switching frequency.
In addition, while the time constant circuit is designed to take into account the dead time, since a dead time having the same time length is set in a full load region, namely even in a transient state and a steady state, it is difficult to set an optimal dead time in the steady state in which a dead time as long as that in the transient state is not necessary. Accordingly, the above-described switching power supply apparatus is less than optimal in terms of efficiency but has a high degree of reliability.